The present invention relates to a device for detecting a property of a liquid such as a fuel containing a plurality of liquid components in a contactless manner. More particularly, it relates to a device for detecting the contents of liquid components such as gasoline, alcohol, etc. contained in a fuel as used with automotive engines.
In recent years, a fuel comprising gasoline mixed with alcohol has become popular for automotive use in many countries including the United States of America, European countries, etc., for the purpose of reducing the consumption of petroleum.
If, however, such an alcohol-mixed fuel is used for engines suited to a gasoline fuel which forms an air fuel mixture having a stoichiometric air/fuel ratio for proper combustion, the air/fuel ratio of a mixture formed of the alcohol-mixed fuel becomes leaner than that with the gasoline fuel due to the fact that the stoichiometric air/fuel ratio is much lower with a fuel containing alcohol than with a gasoline fuel containing no alcohol. For this reason, the content of alcohol in an alcohol-mixed fuel is detected so that engine control elements such as a fuel injector and the like are controlled in accordance with the alcohol content thus detected to properly adjust the air/fuel ratio, ignition timing, etc.; so as to provide good combustion.
Now, a typical example of a conventional fuel property detecting device will be described below.
FIG. 6 shows the general arrangement of a fuel property detecting device disclosed in Japanese Utility Model Laid-Open No. 62-81064. In this figure, the conventional fuel property detecting device includes a refractive index sensor, which is generally designated by reference numeral 101, for sensing the refractive index of a liquid fuel in a contactless manner, a refractive index calculator 102 for calculating the refractive index of the fuel based on the output signal of the sensor 101, a temperature sensor 103 for sensing the temperature of the fuel in the refractive index sensor 101 and generating a corresponding output signal, and an alcohol content calculator 104 for calculating the content of an alcohol contained in the fuel.
As shown in detail in FIG. 6, the refractive index sensor 101 includes a casing 115 at opposite ends of which a light emitter 111 and a light receiver 113 are disposed in an opposed, face-to-face relation so that light 117 emitted from the light emitter 111 passes through a cylindrical light guide 112 towards the light receiver 113.
The casing 115 has a hollow interior in the form of a fuel passage 116, an inlet port 118 for introducing a liquid fuel into the fuel passage 116, and an outlet port 119 for discharging the fuel from the fuel passage 116 to the outside. Thus, a fuel enters the casing 115 from the inlet port 118, flows around the cylindrical light guide 112 in the flow passage 116, and exits the casing 115 from the outlet port 119.
The outer peripheral surface of the cylindrical light guide 112 is sealingly supported at its opposite ends by the opposite end walls of the casing 115 through a pair of annular seals 114 which serve to prevent the leakage of fuel from the interior of the casing 115 towards the outside through the outer periphery of the light guide 112 and the opposite end walls of the casing 115.
The refractive index calculator 102 is connected to the light emitter 111 and the light receiver 113 for calculating the refractive index of the fuel in the fuel passage 116 in the casing 115 based on the output signal from the light receiver 113 and generating a corresponding output signal to the alcohol content calculator 104. Specifically, the refractive index calculator 102 calculates the refractive index of the fuel on the basis of a change or difference between the amount of light emitted from the light emitter 111 and that received by the light receiver 113.
The temperature sensor 103 in the form of a thermistor is mounted on the casing 115 for sensing the temperature of the fuel in the fuel passage 116 in the casing 115 and generating a corresponding output signal to the alcohol content calculator 104.
Based on the output signal of the refractive index calculator 102 and the output signal of the temperature sensor 103, the alcohol content calculator 104 calculates the content of an alcohol contained in the fuel in the fuel passage 116.
FIG. 7 shows the output characteristic of the refractive index calculator 102, and FIG. 8 shows the relationship between the alcohol content and the refractive index at a temperature of 20.degree. C. in which a fuel whose refractive index is to be detected comprises regular or premium gasoline and an alcohol in the form of methanol admixed thereto.
The operation of the above-described fuel property detecting device will be described below. As shown in FIG. 6, the light emitter 111 emits beams of light 117 into the cylindrical light guide 112 at a large conical angle, which are refracted at the interface or boundary surface between the fuel, whose refractive index is NDf, in the fuel passage 116 in the casing 115 and the outer peripheral surface of the cylindrical light guide 112, whose refractive index is NDr, at angles of refraction which depend on the angles of incidence of the respective light beams 117. Thus, part of the light 117 from the light emitter 111 is refracted at the boundary surface and enters the body of fuel in the fuel passage 116, whereas the remaining portion of the light 117 is reflected at the boundary surface into the interior of the cylindrical light guide 112 and received by the light receiver 113.
In this regard, the critical or minimum angle of incidence, at which the light beams 117 from the light emitter 111 incident to the boundary surface are totally reflected into the interior of the cylindrical light guide 112, is called the angle of total reflection .theta.r, and there is the following relationship between the angle of total reflection .theta.r and the refractive indexes NDf, NDr of the fuel and the light guide 112: EQU sin .theta.r=NDf/NDr
Therefore, all the light beams 17 having angles of incidence greater than the angle of total reflection .theta.r are reflected at the boundary surface into the interior of the light guide 112 and received by the light receiver 113.
The refractive index NDf of the alcohol-mixed fuel varies in accordance with the content of alcohol Cm therein, so the angle of total reflection .theta.r accordingly changes with the alcohol content Cm. Thus, the amount of light received by the light receiver 113 changes in dependence upon the alcohol content Cm in the fuel. For this reason, the light receiver 113 comprises an element such as a phototransistor which generates an electric current having a magnitude proportional to the amount of light received. The current thus generated is input to the refractive index calculator 102 where it is converted into a corresponding voltage which is proportional to the amount of light received by the light receiver 113.
Now, let us consider the case in which the fuel to be detected comprises a gasoline in the form of regular gasoline mixed with methanol, and the cylindrical light guide 112 is formed of an optical glass BK7 having a refractive index of 1.52. In this case, as clearly shown in FIG. 8, the angle of total reflection .theta.r of regular gasoline (i.e., a fuel comprising regular gasoline containing no methanol (MO)) at room temperature, which has a refractive index of about 1.42, is about 69 degrees, whereas that of methanol (i.e., a fuel comprising 100% methanol containing no gasoline (M100) at room temperature, which has a refractive index of 1.33, is 49 degrees. As seen from FIG. 8, the higher the alcohol content Cm in regular gasoline, the lesser the refractive index NDf of the alcohol-mixed fuel and hence the lesser the angle of total reflection .theta.r becomes. Therefore, as the alcohol content Cm in regular gasoline increases, beams of light 117 projected from the light emitter 111 at an increasing conical angle of projection can reach the light receiver 113, so the amount of light received by the light receiver 113 increases. As a result, the output VND of the refractive index calculator 102 decreases in inverse proportion to the increasing refractive index NDf of the fuel, as clearly seen from FIG. 7.
Since the alcohol content Cm in the fuel is inversely proportion to the refractive index NDf thereof, as shown in FIG. 8, the alcohol content calculator 104 calculates, based on the output VND of the refractive index calculator 102, the alcohol content Cm and generates a corresponding output signal. In this case, however, the refractive index ND of the fuel varies with its temperature, i.e., in inverse proportion to the temperature thereof. Accordingly, the temperature sensor 103 senses the temperature Tf of the alcohol-mixed fuel and generates a corresponding output to the alcohol content calculator 104 which modifies, on the basis of the fuel temperature Tf, the alcohol content Cm, which is previously calculated from the output VND of the refractive index sensor 2, to provide a temperature-compensated correct alcohol content VCm.
With the above-described conventional device, however, in the case of a mixed fuel comprising a plurality of kinds of gasoline admixed with an alcohol such as, for example, one consisting of regular gasoline, premium gasoline and an alcohol, there will be an error in the alcohol content VCm calculated in the above manner, which can become .DELTA.Cm at the greatest, as shown in FIG. 8. This is because there is a difference in the refractive index between regular gasoline and premium gasoline.
In addition, there is a variation in the temperature dependency of the refractive indexes of various kinds of fuels or different fuel components, so it is extremely difficult to exactly detect the content of an alcohol or a liquid component in a fuel mixture over a variety of kinds of fuels.
As a consequence, in cases where a mixed fuel comprising regular gasoline and an alcohol is admixed with premium gasoline, it becomes almost impossible to properly control the engine by accurately adjusting the air/fuel ratio of a mixture supplied to the engine, ignition timing, the amount of fuel injection, and the like.